Ion implantation is a process used in the manufacture of semiconductor devices. Typically, ionized dopants, such as A.sub.S H.sub.3 (arsine) and PH.sub.3 (phosphine), are implanted into the surface of a masked wafer by accelerating them at high speed. Once the dopant is implanted, it can be diffused to desired regions of the semiconductor device. A typical ion implanter includes an ion source for producing ionized dopant, a dopant energizer, a mounting system for mounting the wafer, and an exhaust system for exhausting gases remaining in the cavity of the ion implanter after the completion of the ion implantation process.
A typical exhaust system includes an exhaust pump that is in communication with the cavity of the ion implanter. An exhaust line is connected at one end to the exhaust pump and at its other end to a main exhaust duct located outside of the ion implanter. The exhaust line is generally formed of polyvinyl chloride (PVC), a material that can withstand the high voltages applied to the ion implanter during processing because of its high dielectric strength.
One problem that plagues ion implanters that employ PVC as the material of the exhaust line is the adherence of contaminating by-products created during processing to the inner surface of the exhaust line. Typically, the exhaust pump is formed of steel, so some iron from the surface of the pump diffuses into the exhaust stream. Also, moisture (in the form of steam) is typically present in the exhaust. The water can oxidize the iron to cause iron oxide (rust) to form on the walls of the PVC. Once iron builds up on the inner surface of the exhaust line, moisture within the exhaust tends to condense onto the iron. Also, hot by-products from the implantation process, which typically include arsine, phosphine, or both, can be oxidized by excess moisture as demonstrated in the following reactions: EQU 2AsH.sub.3 +5H.sub.2 O=As.sub.2 O.sub. 5 +8H.sub.2 ( 1) EQU 2PH.sub.3 +5H.sub.2 O=P.sub.2 O.sub.5 +8H.sub.2 ( 1)
Attachment of these compounds to the inner wall of the exhaust line can reduce the dielectric strength of the exhaust line significantly.
The reduction of the dielectric strength of the exhaust line and the presence of iron thereon can make the exhaust line susceptible to a the generation of a corona discharge inside the exhaust line when a high voltage is applied to the implanter. Such a corona discharge can be deleterious to the fabrication of semiconductor devices within the implanter. Moreover, because the reactions set forth above produce hydrogen gas as a reaction product, a corona discharge can ignite the hydrogen and burn the PVC of the exhaust line.